Drug delivery systems for the treatment of disease which introduce active ingredients into the circulation are numerous and include oral, transdermal, subcutaneous and intravenous administration. While these systems have been used for quite a long time and can deliver sufficient medication for the treatment of many diseases, there are numerous challenges associated with these drug delivery mechanisms. In particular, delivery of effective amounts of proteins and peptides to treat a target disease has been problematic. Many factors are involved in introducing the right amount of the active agent, for example, preparation of the proper drug delivery formulation so that the formulation contains an amount of active agent that can reach its site(s) of action in an effective amount.
The active agent must be stable in the drug delivery formulation and the formulation should allow for absorption of the active agent into the circulation and remain active so that it can reach the site(s) of action at effective therapeutic levels. Thus, in the pharmacological arts, drug delivery systems which can deliver a stable active agent are of utmost importance.
Making drug delivery formulations therapeutically suitable for treating disease depends on the characteristics of the active ingredient or agent to be delivered to the patient. Such characteristics can include, in a non-limiting manner, solubility, pH, stability, toxicity, release rate, and ease of removal from the body by normal physiologic processes. For example, in oral administration, if the agent is sensitive to acid, enteric coatings have been developed using pharmaceutically acceptable materials which can prevent the active agent from being released in the low pH (acid) of the stomach. Thus, polymers that are not soluble at acidic pH are used to formulate and deliver a dose containing acid-sensitive agents to the small intestine where the pH is neutral. At neutral pH, the polymeric coating can dissolve to release the active agent which is then absorbed into the systemic circulation. Orally administered active agents enter the systemic circulation and pass through the liver. In certain cases, some portion of the dose is metabolized and/or deactivated in the liver before reaching the target tissues. In some instances, the metabolites can be toxic to the patient, or can yield unwanted side effects.
Similarly, subcutaneous and intravenous administrations of pharmaceutically-active agents are not devoid of degradation and inactivation of the active ingredients. With intravenous administration of drugs, the drugs or active ingredients can also be metabolized, for example in the liver, before reaching the target tissue. With subcutaneous administration of certain active agents, including various proteins and peptides, there is additionally degradation and deactivation by peripheral and vascular tissue enzymes at the site of drug delivery and during travel through the venous blood stream. In order to deliver a dose that will yield an acceptable quantity for treating disease with subcutaneous and intravenous administration of an active agent, dosing regimes will always have to account for the inactivation of the active agent by peripheral and vascular venous tissue and ultimately the liver.